This invention relates to an acoustic source for sensing the properties of a ground formation, particularly a rock formation during drilling or other operation within a wellbore.
The co-owned United States Patent Application Publication No. US 2003/0205428 A1, the disclosure of which is incorporated herein by reference, describes an acoustic source using a solenoid which is energized by an electric current pulse to cause a metal hammer to be propelled, by electromagnetic force, towards a bore hole wall. Such an electromagnetically actuated hammer is known to be an extremely effective acoustic source when the hammer is fired against a rock formation or against a metal plate (serving as an anvil) placed in firm contact with the rock formation.
In use, the acoustic source is mounted on drilling apparatus (e.g. the bottom hole assembly of a drill string pipe, drill-collar or drill bit) and a major problem is protecting the components of the acoustic source, notably the moveable hammer, from the surrounding environment (which contains drilling mud and cuttings) and from wear on the hammer resulting from the rotational movement of the bottom hole assembly, collar or bit. The invention aims to provide an acoustic source which overcomes this problem.